1. Field of the Invention
This invention relates to an image transmitting apparatus and, more particularly, to an image transmitting apparatus for digitizing a still-picture television signal, a manuscript image signal or the like and transmitting the digitized signal over a line.
2. Description of the Prior Art
A still-picture transmitting apparatus has recently been proposed and developed in which one frame of a still-picture video signal reproduced from a floppy disk storing video data is temporarily stored in a memory, and the picture signal is successively read out of the memory and transmmitted through a line in conformance with the transmission speed. Most transmitting apparatus of this kind employ digital transmission owing to the recent popularization of transmission by personal computer, the higher speed of modem devices and the lower cost thereof. Some of these transmitting apparatus are capable of transmitting a color picture and generally do so using RGB or Y and color difference signals (R-Y), (B-Y) as color information. If the transmission protocols are made to conform, such an apparatus is capable of transmitting a picture signal also to a bi-level facsimile machine, which is becoming increasingly popular.
In such case, it is necessary to convert a color television signal, in which one pixel (picture element) ordinarily is expressed in 256 tones, into a black-and-white binary signal. The simplest method of achieving this binary conversion entails comparing a luminance signal Y with the central level (127) of the 256 tones, and making the signal "1" when it is above the central level and "0" when it is below the central level, thereby deciding black or white, by way of example. A common binary coding method is the dither processing method, in which binary conversion is performed by making, e.g., a 4.times.4 dither matrix correspond to each pixel level obtained by sampling the television signal.
The number of pixels in the main scanning direction of common facsimile machines is set in conformity with paper size by the CCITT advisory. For instance, the figure is 1728 dots in the case of size A4. On the other hand, the number of items of pixel data according to television standards is unrelated to paper size. For example, consider a case where a video memory storing a television signal is composed of 640 (pixels).times.480 (lines), and binary coding according to the dither method is adopted by employing the vertical direction of data in the video memory as the main scanning direction. If the size of the dither matrix in such case is 4.times.4, then the number of dots in the main scanning direction of the digitized image data will be 480.times.4=1920 (dots). As mentioned above, the pixel size of A4 paper is 1728. Therefore, in order to print a television picture signal on size A4 paper, it is necessary to discard the extra 198 dots (1920-1728=198). If the dither matrix is a 3.times.3 matrix, then the number of dots in the main scanning direction will be 480.times.3=1440 (dots), which will fit into the size A4 paper width (1728 dots). However, this means that approximately 12% of the paper area will be wasted. Another problem is that tone expression is diminished by reducing the 4.times.4 dither matrix to the 3.times.3 dither matrix. These problems are not limited solely to binary coding using the dither method. The same problems arise also when performing binary coding by the error diffusion method or dot pattern method, and when simply binary coding and transmitting image data which conform to television standards, by way of example.